R. T. C. Brownlee

Applications of molybdenum-95 NMR spectroscopy. 14. Construction of a transverse probe for the detection of broad 95Mo resonances in solution. Studies on Binuclear Mo(II) compounds

The use of a specifically constructed probe having a transverse solenoid coil operating at 13 MHz has permitted convenient detection of 95Mo resonances in solutions of [Mo] = 5 × 103 M for compounds whose linewidths, W12, are in excess of 1000 Hz. Application of a three-pulse sequence, 90°+x (acq)+-180°x-τ-90°−x (acq)+ and shielding of the coil with lead substantially reduces ring-down times. A series of binuclear molybdenum(II) species which feature formal Mo-Mo quadruple bonds were examined. 95Mo resonances were detected in the range 3227–4148 ppm and this is the most deshielded class of molybdenum compounds so far discovered. Observed linewidths covered the range 200–1500 Hz. Those for the compounds [MO2(O2CR)4] could be rationalized on the basis of quadrupolar relaxation and the Hu-Zwanzig model of molecular motion.

Molybdenum-95 nuclear magnetic resonance. Applications to substituted carbonyls

Molybdenum-95 NMR spectra of a variety of substituted molybdenum carbonyl species are reported. The large chemical shift range permits easy resolution of substituent effects, both within similar ligands and in the number of carbonyls replaced. Molybdenum-95—phosphorus-31 spin—phosphorus-31 spin—spin coupling is observed by this technique for the first time. The results are discussed in terms of general applications of the technique to molybdenum organometallic chemistry.

Carbon-13 N.M.R. Studies of Carbocations. III. An Investigation of the Variation of ipso-Substituent Chemical Shifts with Electron Demand in 1,4-Disubstituted Benzenes

13C n.m.r. spectra have been obtained of a large range of 1(X),4(Y)-disubstituted benzenes in which X has been varied over a range of 25 substituents from NMe2 to +CHMe for each of the compounds where Y = H, OMe, Me, F, Cl, Br and CF3. The ipso-substituent chemical shifts (ipso-SCS) for each of the latter (that is, the change in chemical shift (Δδ) of C4 by replacement of H by Y) have been shown to vary dramatically with the electron demand of the X substituent as measured by δ(C4). When plotted against &3948;(C4), the ipso-SCS of F and OMe both decrease linearly with increasing electron demand whilst those of Br and Cl show linear increases. Those of Me and CF3 show discontinuities which indicate changes in the mechanism of interaction of these groups with the attached ipso-carbon. The variations in the ipso-SCS with electron demand of X are considered to be due to Y-induced variations in the sensitivity of the ipso-carbon to the effect of the para (X) substituent and not to through-conjugation effects. The results clearly show the fallacy of assuming that 13C substituent effects are constant.